In the currently leading developing method using a two-component dry developer comprising a toner and a carrier, the toner is consumed by development whereas the carrier is not consumed and remains in a development unit. As a result, a toner component adheres to the carrier particles to contaminate the carrier. Further, the carrier itself is under the stress of collision in the developing unit. As a result, the resin coat eventually comes off the carrier particles to influence the developer characteristics such as chargeability. This causes a fluctuation in the toner image density or causes generation of fog.
So-called trickle type developing equipment has been proposed to solve the above problem as disclosed in JP-B-2-21591, in which a carrier trickles into the development unit together with a toner that is to be consumed for development to slowly exchange the carrier in the developing unit for a fresh one, to thereby suppress the variation in charge quantity and to stabilize the image density. However, since the trickling carrier is the same as that previously contained in the developing unit, the deteriorated carrier increases while development operation is repeated, resulting in a failure to suppress the variation of image density.
JP-A-3-145678 proposes use of a toner in combination with a carrier having a higher resistivity than the carrier previously present in a developing unit to thereby maintain chargeability and suppress reduction in image quality. According to this technique, however, the amount of the carrier to be fed changes according to the amount of the toner consumed so that the resistance of the developer in the developing unit varies, resulting in variation of image density.
JP-A-8-234550 discloses a method in which a plurality of toners for replenishment which contain a carrier having different physical properties from those of the carrier previously contained in the developing unit are fed successively. However, because the carrier and the toner differ extremely in specific gravity, it is practically very difficult to replenish the developing unit successively with the replenishing toners using a single feed container while avoiding mixing up of the replenishing toners. In addition, the carrier is liable to deterioration due to the large proportion of the toner over the carrier, resulting in a failure to obtain a stable image over an extended period of time.